What is pleural effusion

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"pleural effusion"[MeSH Terms] AND diagnosis

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Here is a thorough overview of pleural effusion, drawing from ROSEN's Emergency Medicine, Murray & Nadel's Textbook of Respiratory Medicine, and Grainger & Allison's Diagnostic Radiology.

Pleural Effusion

Definition

Pleural effusion is an abnormal collection of fluid in the pleural space - the potential space between the visceral and parietal pleura. Under normal conditions, a thin layer of fluid (~0.1-0.2 mL/kg body weight, about 5-15 mL total) maintains dynamic equilibrium, with approximately 1 L traversing the pleural space every 24 hours. An effusion develops when fluid influx exceeds efflux.

ROSEN's Emergency Medicine, p. 988

Anatomy and Physiology of Fluid Balance

Pleural fluid is produced from systemic capillaries at the parietal pleural surface and absorbed into pulmonary capillaries at the visceral pleural surface. The direction of flow is governed by the difference in hydrostatic pressures between the systemic and pulmonary circulations, with lymphatics also assisting drainage.

Pressures involved in pleural fluid formation and absorption - hydrostatic vs. colloid osmotic pressures across chest wall and lung

Diagrammatic representation of the pressures involved in pleural fluid formation and absorption - ROSEN's Emergency Medicine

Causes

Pleural effusions are divided into transudates and exudates based on fluid composition.

Transudates (ultrafiltrates of plasma; low protein)

Caused by increased hydrostatic pressure or decreased oncotic pressure:

Congestive heart failure (most common cause overall)
Cirrhosis with ascites
Nephrotic syndrome
Hypoalbuminemia / severe malnutrition
Myxedema
Peritoneal dialysis
Superior vena cava obstruction
Pulmonary embolism (can be transudative or exudative)

Exudates (high protein; reflects intrinsic pleural abnormality)

Caused by inflammation, increased permeability, or lymphatic obstruction:

Infections: bacterial pneumonia (parapneumonic effusion), TB, lung abscess, bronchiectasis, viral illness
Malignancy: primary lung cancer, mesothelioma, metastases, lymphoma
Connective tissue disease: rheumatoid arthritis, SLE
Abdominal/GI disorders: pancreatitis, subphrenic abscess, esophageal rupture
Miscellaneous: pulmonary infarction, uremia, chylothorax

Key terminology: A parapneumonic effusion is one associated with bacterial pneumonia or lung abscess. Empyema (pyothorax) means pus in the pleural space. A loculated effusion is fluid confined by adhesions between the visceral and parietal pleura.

ROSEN's Emergency Medicine, p. 988-989

Pathophysiology

Type	Mechanism	Common Causes
Transudate	High hydrostatic pressure OR low oncotic pressure	Heart failure, cirrhosis, nephrotic syndrome
Exudate	Pleural/pulmonary inflammation, increased capillary permeability, lymphatic obstruction	Pneumonia, malignancy, PE (often mixed)

Massive effusions (>1.5-2 L) are usually malignant but can occur in heart failure and volume overload states. They can restrict respiratory movement, compress lung parenchyma, cause intrapulmonary shunting, and in extreme cases lead to tension hydrothorax with mediastinal shift and circulatory compromise.

Clinical Features

Small effusions: typically asymptomatic
Moderate-large effusions: dyspnea (usually does not develop until >500 mL fluid accumulates), cough
Pleuritic pain: sharp chest pain worse with deep breathing; may radiate to ipsilateral shoulder (indicates pleural inflammation)

Physical examination findings:

Diminished or absent breath sounds
Dullness to percussion
Decreased tactile fremitus
Pleural friction rub (with pleuritis)
Hemodynamic compromise with massive effusions

Diagnosis

Imaging

Chest X-ray:

Requires ~200 mL to be visible on upright frontal view
Classic finding: blunting of the costophrenic angle
Larger effusions obscure the hemidiaphragm with an upward concave ("meniscus") appearance - fluid layers higher laterally than centrally
Massive effusion can completely opacify the hemithorax

Radiograph of a right-sided pleural effusion - ROSEN's Emergency Medicine

CT scan: Gold standard for small effusions; detects as little as 3-5 mL. Useful for distinguishing pleural from parenchymal disease and detecting underlying causes.

Ultrasound: Can detect as little as 50 mL. More sensitive than chest X-ray. Simple transudates appear hypoechoic; complex/exudative effusions may show heterogeneous echogenicity, septations, or loculations. Ultrasound guidance significantly reduces the risk of iatrogenic pneumothorax during thoracentesis.

Thoracentesis and Fluid Analysis

Most patients with a new pleural effusion should undergo diagnostic thoracentesis (can be deferred if an obvious cause like heart failure is clear).

Light's Criteria - the most widely accepted method for differentiating transudates from exudates. Fluid is an exudate if ANY ONE of the following is present:

Criterion	Threshold
Pleural fluid protein / serum protein	> 0.5
Pleural fluid LDH / serum LDH	> 0.6
Pleural fluid LDH	> 2/3 of upper limit of normal serum LDH

Additional markers:

pH < 7.3: suggests parapneumonic effusion, malignancy, rheumatoid effusion, TB, or systemic acidosis
Glucose, cell count, differential, culture, cytology ordered as clinically indicated
ROSEN's Emergency Medicine, p. 989-990

Management

Management depends entirely on the underlying cause:

Cause	Treatment
Heart failure	Diuresis (IV if severe)
Parapneumonic/empyema	Antibiotics + early drainage; empyema benefits from surgical drainage
Malignant effusion	Therapeutic thoracentesis; recurrent cases require pleurodesis or indwelling pleural catheter
Hepatic hydrothorax	Sodium restriction + diuretics; refractory cases: TIPS, VATS repair of diaphragmatic defects, or indwelling catheter as bridge to transplant
Viral pleuritis	Supportive (self-limited)
TB effusion	Anti-TB chemotherapy

Conventional chest tube drainage is not recommended for hepatic hydrothorax due to risk of protein/electrolyte loss, infection, and renal failure.
In ~20% of effusions, no definitive diagnosis is established; most resolve spontaneously.

Summary

Pleural effusion is a common clinical finding with a broad differential. The key diagnostic step is classifying the fluid as a transudate vs. exudate using Light's criteria, guided by clinical context. Ultrasound has become the standard for both diagnosis and procedure guidance. Treatment targets the underlying cause.

Sources: ROSEN's Emergency Medicine Concepts and Clinical Practice; Murray & Nadel's Textbook of Respiratory Medicine

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